1. Field of Invention
The present invention relates to an organic light-emitting device. In particular, the invention relates to an organic electroluminescence (EL) or photoluminescence (PL) device that has higher external quantum efficiency by forming a composite film layer with a gradually varying refraction index between an ITO transparent electrode and a transparent substrate.
2. Related Art
After the first high efficiency green-light organic electroluminescence (EL) device developed by Kodak in 1987 (see the U.S. Pat. No. 4,885,211), organic EL devices have been receiving a lot of attention. The action principle of the organic EL devices is to sandwich an organic thin film between an anode and a cathode so that oppositely charged carriers (electrons and holes) will be driven by a electric field and meet in the organic layer and emit photons.
The structure of a simple two-layer organic EL device is shown in FIG. 1. A transparent ITO conduction electrode 20 (which is the anode with a thickness of 30 nm to 400 nm and a surface resistance less than 100 xcexa9/cm2) is coated on the surface of a plastic (acrylic, epoxy, polyethylene terephthalate (PET), polycarbonate (PC), etc) or glass substrate 10 as a hole injection electrode. One or several organic light-emitting layers 30 are then coated onto the transparent ITO conduction electrode 20. Another metal conduction electrode 40 (which is the cathode of about 100 nm to 400 nm thick and contains such metals as Mg, Li, Al, Ca, Ag, In and their alloys) is further coated onto the organic light-emitting layer 30 as an electron injection electrode layer. Owing to the difference in materials used for the organic light-emitting layer 30, the organic EL devices can be roughly classified into molecule-based organic EL devices and polymer-based organic EL devices. The coating methods of the organic light-emitting layer for these two types of organic EL devices are different. The former usually has a double layer structure. For example, the technology disclosed in the U.S. Pat. No. 5,844,363 uses a hole transport layer, N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-(m-tolyl) benzidine (TPD), and an electron transport layer, tri-(8-hydroquinoline) aluminum (Alq3), each with a thickness of about 80 nm. The two layers are coated on the ITO film by vacuum vapor deposition. The polymer-based organic EL device uses a single layer of poly(2-methoxy-5-(2xe2x80x2-ethyl-hexyloxy)-p-phenylene vinylene (MEH-PPV) with a thickness of about 50 nm to 100 nm. This layer is coated on the ITO or a camphor sulfonic acid (CSA) doped polyaniline (PANI) film by spin coating, soak coating, scraping, screening or ink jet printing (G. Gustafson et al., xe2x80x9cThe xe2x80x98plasticxe2x80x99 LED: a flexible light-emitting device using a polyaniline transparent electrode,xe2x80x9d Synthetic Metals 55-57, 4123-4127(1993)).
As indicated in the report by G. Gu et al. in xe2x80x9cHigh-external-quantum-efficiency organic light-emitting devices,xe2x80x9d Optics Letts., 396-398 (1997), the external quantum efficiency of photoluminescence (PL) or EL by either molecule-based organic EL devices or polymer-based organic EL devices is extremely low (about 17.5%). The main reason is that 51% and 31.5% of the light emitted from the organic EL layer are trapped within the ITO electrode and the glass substrate, respectively. This is caused by the reflection and total reflection due to the refraction index difference between the ITO electrode (n=1.9) and the plastic or glass substrate (n=1.52), where the critical angle for total reflection is 53.1 degrees, and the refraction index difference between the plastic or glass substrate and the air (n=1), where the critical angle for total reflection is 41.1 degrees.
Therefore, how to lead the light from the interior of a molecule-based organic EL device or polymer-based organic EL device to outside so as to increase the external quantum efficiency of the PL or EL, and thus lowering the power consumption and increase the product lifetime, is a big research subject in the field.
In the U.S. Pat. No. 5,834,893 (V. Bulovice et al., xe2x80x9cHigh efficiency organic light emitting devices with light directing structuresxe2x80x9d), the disclosed technology is to make reflective mirrors with different shapes on the surface of a plastic or glass substrate. The reflective light trapped in the molecule-based organic EL layer, the polymer-based organic EL layer, or the ITO transparent conduction layer can escape out of the device surface because the reflected light has an incident angle smaller than the critical angle for total reflection at the interface. Nevertheless, the problem is such geometrical structures will change the incident (reflective) angle of the excited light. Therefore, the efficiency is limited and the emitted light may have an inhomogeneous intensity distribution.
It is a primary objective of the present invention to provide an organic electroluminescence (EL) or photoluminescence (PL) device with high external quantum efficiency.
It is another objective of the present invention to provide a photon wave guide device that can improve the external quantum efficiency of organic EL or PL devices and the corresponding structure.
The technologies proposed herein try to solve the problem of inferior external quantum efficiency of organic light-emitting devices with a transparent ITO electrode and using glass or plastic as the substrate material. To avoid the reflections at the interface between the ITO electrode and the plastic or glass substrate for the light emitted from the organic EL layer and the luminescence loss due to the reflection at the interface between the plastic or glass substrate and the air, the present invention inserts a composite film layer with a gradually varying refraction index between the ITO electrode and the plastic or glass substrate. The composite film layer is made of materials with two different refraction indices so that the refraction index change is compatible with the ITO electrode and the plastic or glass substrate. In other words, the refraction index of the composite film layer is the same as that of the ITO electrode (n=1.9) at their interface and gradually changes along the direction perpendicular to the substrate in a linear or nonlinear way (such as a stair-shape curve). The refraction index of the composite film layer finally becomes the same as that of the substrate (n=1.49 to 1.6 for plastic and n=1.52 for glass) at their interface. This design can avoid reflection due to the refraction index difference across the interface. Accordingly, the excited radiation originally trapped in the ITO electrode can then be led outside the device.
In another preferred embodiment of the invention, a photon wave guide is provided at the interface between the substrate and the air to increase the external quantum efficiency of such organic EL or PL devices.